Dual tone multi-frequency (DTMF) signals were originally developed to facilitate the automated dialing of telephone numbers by end-users. They have since found wider application in transmitting other information, such as for example in voice mail systems and automated telephone menu systems. In the public switched telephone network (PSTN), DTMF signals are transmitted in-band with voice signals. A set of standards have been developed to set specifications for how DTMF is to be generated and when a DTMF tone should be recognized. For example the International Telecommunications Union has developed standards ITU-T Q.23 and ITU-T Q.24.
A DTMF signal is a sum of two predetermined frequencies, one selected from a set of four low frequency tones, and one selected from a set of four high frequency tones. Each pair of tones represents one of sixteen keys. The standard telephone keypad only uses the first twelve keys, and not the four alphanumeric keys (A to D), which are reserved for future use. The frequencies and their corresponding keys are outlined below in Table 1.
TABLE 1f(Hz)1209133614771633697123A770456B852789C941*0#D
Digital packet-based networks, like the Internet, are increasingly being used for transmission of telephone signals. The use of packet-based networks for telephony presents a problem for DTMF signaling. The compression techniques used for preserving bandwidth in packet-based networks are acceptable for speech, since the associated degradation is tolerable; however, these compression techniques degrade DTMF signals to an extent that they no longer satisfy the standards and will fail to be recognized at the receiver.
In order to address this problem, packet-based networks mat attempt to detect DTMF tones in a voice signal so as to strip them out before transmitting the voice signal. The DTMF tones represent alphanumeric information, and can easily and more efficiently be sent as a separate binary signal, or be otherwise encoded in the packetized linear voice signal. Accordingly, a packet-based network may employ a DTMF detector.
Another difficulty encountered with packet-based signals is that of end-to-end delay. Delay results from the processing associated with packetization and compression, and from the inherent delay in transmission through routers. Delay in packet-based telephony is particularly noticeable to the users.
Detecting a DTMF tone takes a certain amount of time, and the lag associated with this detection time can result in a portion of the DTMF tone leaking in-band prior to the detection occurring and prior to triggering a muting of the DTMF tone. At the receiving end, this leaked portion will be recombined with a regenerated DTMF signal and can result in spectral line splitting, which in turn can result in a failure to detect a valid DTMF digit. In some cases, it can lead to the false detection of double digits.
To avoid the spectral line splitting problem, a transmission delay must be incorporated so as to hold the voice packet while the DTMF detector attempts to detect a DTMF signal. The voice packet is only sent once the DTMF detector has finished analyzing whether or not the packet contains a DTMF signal. Therefore, DTMF detectors having a significant delay will further aggravate the delay problems of packet-based telephony.
Existing DTMF detectors typically require at least 10 milliseconds to analyze a packet for DTMF signals.
Accordingly, there remains a need for a DTMF detector that is capable of detecting DTMF signals quickly.